As compared with CRT (cathode ray tube), a liquid crystal display (LCD) is advantageous in that it has a small size and a light weight and consumes a low electric power. The liquid crystal display comprises a liquid crystal cell and a pair of polarizing plates disposed on the respective side of the liquid crystal cell. The liquid crystal cell comprises rod-shaped liquid crystal molecules, two sheets of substrates for enclosing the rod-shaped liquid crystal molecules therein and an electrode layer for applying a voltage to the rod-shaped liquid crystal molecules. In order to align the rod-shaped liquid crystal molecules thus enclosed in the liquid crystal cell, the two sheets of substrates each have an alignment film provided thereon. In order to decolor the image displayed on the liquid crystal cell, it is often practiced to provide an optical compensation film (retarder plate) interposed between the liquid crystal cell and the polarizing plate. The layered product of the polarizing plate (polarizing film) and the optical compensation film acts as an elliptical polarizing plate. The optical compensation film may be rendered capable of expanding the viewing angle of the liquid crystal cell. A stretched birefringence film has heretofore been used as an optical compensation film.
It has also been proposed that an optical compensation film having an optical anisotropic layer containing a discotic compound be used instead of stretched birefringence film (see, e.g., JP-A-6-214116, U.S. Pat. No. 5,583,679, German Patent Application Disclosure 3,911,620 and U.S. Pat. No. 4,583,825). The optical anisotropic layer is formed by aligning a discotic compound, and then fixing the discotic compound thus aligned. A discotic compound normally has a high birefringence. A discotic compound can be aligned in various forms. Accordingly, a discotic compound can be used to produce an optical compensation film having optical properties that have never been obtained with the prior art stretched birefringence films.
The liquid crystal cell performs ON/OFF display depending on the difference in orientation of liquid crystal compound. Display modes such as TN (Twisted Nematic), IPS (In-Plane Switching), OCB (Optically Compensatory Bend), VA (Vertically Aligned), ECB (Electrically Controlled Birefringence) and HAN (Hybrid Alignment Nematic), which can be applied to both transmission and reflection type display devices, have been proposed.
As those requiring a high fidelity display among these LCD's there are mainly used 90° twisted nematic liquid crystal displays (hereinafter referred to as “TN mode”) comprising a nematic liquid crystal compound having a positive dielectric anisotropy which are driven by a thin-film transistor. These TN mode liquid crystal displays exhibit excellent display properties as viewed on the front but are disadvantageous from the standpoint of display properties in that they exhibit a lowered contrast as viewed obliquely or gradation inversion during gradation display. It has been keenly desired to provide improvements in these display properties. These TN mode liquid crystal displays are also disadvantageous in that they have limited response. It has thus been desired to develop LCD comprising a liquid crystal mode having a higher response.
Optical compensation films have been heretofore developed for TN mode liquid crystal displays. With the recent increasing demand for liquid crystal television, it has been pointed out that the liquid crystal displays leave something to be desired in response, e.g., trailing animation, afterimage. Therefore, OCB mode (or bend mode) liquid crystal displays characterized by high response have been noticed. For example, U.S. Pat. No. 4,583,825 and U.S. Pat. No. 5,410,422 disclose a liquid crystal display comprising a bend-aligned liquid crystal cell having rod-shaped liquid crystal molecules aligned in directions which are opposite from upper to lower side of the liquid crystal cell (symmetrically). Since the rod-shaped liquid crystal molecules are symmetrically aligned in directions which are opposite from upper to lower side of the liquid crystal cell, the bend-aligned liquid crystal cell has a self optical compensating function. Therefore, this liquid crystal mode is also called OCB (optically compensatory bend) liquid crystal mode. The bend-aligned liquid crystal display is advantageous in that it exhibits a high response.
As compared with ordinary liquid crystal modes (TN mode, STN mode), the bend alignment mode is characterized by a great viewing angle and a high response. However, the bend alignment mode needs to be further improved as compared with CRT. In order to further improve the bend-aligned liquid crystal displays, it can be proposed that an optical compensation film be used as in the ordinary liquid crystal modes. However, the prior art optical compensation film comprising a stretched birefringence film leaves something to be desired when used in the bend-aligned liquid crystal display. As previously mentioned, it has been proposed that an optical compensation film having an optical anisotropic layer containing a discotic compound be used instead of stretched birefringence film. For example, the inventions disclosed in JP-A-9-211444 and JP-A-11-316378 concern an OCB mode liquid crystal display comprising an optical compensation film having a liquid crystal compound layer. However, it is difficult to obtain good viewing angle properties merely by controlling known optical parameters as disclosed in these patent references.
Further, a bend-aligned liquid crystal display comprising an optical compensation film containing a discotic compound has been also proposed (see, e.g., JP-A-9-197397 and International Patent Disclosure 96/37804 pamphlet). Moreover, various methods have been proposed to eliminate tin change and prevent gradation inversion of a bend-aligned liquid crystal device comprising an optical compensation film containing a discotic compound (see, e.g., Japanese Patent 3056997 and JP-A-2002-40429).
The use of an optical compensation film containing a discotic compound makes it possible to remarkably improve the viewing angle of a bend-aligned mode liquid crystal display.
The liquid crystal display of bend alignment mode comprising an optical compensation film containing a discotic compound has been claimed disadvantageous in that light having a specific wavelength leaks, causing the coloration of the displayed image (see, e.g., JP-A-11-316378). It is disclosed that this coloration is attributed to the wavelength dependence of the transmission of the elliptical polarizing plate (laminate of polarizing film and optical compensation film). Since the wavelength dependence of the anisotropy of the liquid crystal molecules incorporated in the liquid crystal cell and the wavelength dependence of the anisotropy of the optical compensation film (e.g., discotic liquid crystal) differ from each other, a liquid crystal display of bend alignment normally shows the leakage of short wavelength light (blue). However, the tint during black display can be improved by making the wavelength dispersion of the liquid crystal molecules and the optical compensation film (discotic liquid crystal) coincident with each other.
It has been reported that when the optical anisotropic layer and the polarizing film are disposed in such an arrangement that the angle of the average of direction of line normal to the disc surface of the discotic compounds regularly projected on the optical anisotropic layer and the in-plane transmission axis of the polarizing film with respect to each other is substantially 45°, the maximum optical compensation effect on the liquid crystal cell of bend alignment mode. In order to eliminate tint change and prevent gradation inversion on bend-aligned liquid crystal display comprising an optical compensation film containing a discotic compound, various methods have been proposed (see, e.g., Japanese Patent 3056997 and JP-A-2002-40429).
In recent years, a liquid crystal display has been used in large-sized TV more and more. Therefore, liquid crystal displays having a higher display fidelity have been required. Referring to black display fidelity in particular, it has been difficult to attain a high black display fidelity while satisfying desired contrast properties and viewing angle properties for bend-aligned liquid crystal displays even if the prior art optical compensation film containing a discotic compound is used.
In each publication of JP-A-7-306317 and JP-A-9-104866, there is disclosed 2,3,6,7,10,11-hexa{4-(6-acryloyloxyhexyloxy)benzoyloxy}triphenylene is disclosed as a discotic liquid crystalline molecule suitable for the formation of the optical anisotropic layer of the phase plate (Patent Documents 1 and 2). Incidentally, the retardation value (Δnd) of the phase plate is determined according to the optical properties of the liquid crystal cell to be compensated. The retardation value (Δnd) is the product of the refractive index anisotropy (Δn) of the optical anisotropic layer and the thickness (d) of the optical anisotropic layer. When the refractive index anisotropy (Δn) of the optical anisotropic layer is large, the liquid crystal cell can be compensated even if the thickness (d) of the layer is small. However, with the discotic liquid crystalline compound described in each of Patent Document 3 and Patent Document 4, it is very difficult to form an optical anisotropic layer having a sufficiently large refractive index anisotropy (Δn) (JP-A-7-306317 and JP-A-9-104866). Whereas, JP-A-2001-166147 discloses a discotic liquid crystal having a large refractive index anisotropy. However, the wavelength dispersion characteristic is degraded, (namely, the wavelength dispersibility increases), so that the performance improvement is insufficient (JP-A-2001-166147). In general, the wavelength dispersion characteristic and the refractive index anisotropy are in a relationship of tradeoff. An increase in the refractive index anisotropy results in the degradation of the wavelength dispersion characteristic. The degradation of the wavelength dispersion characteristic degrades the color taste change in color display which is one of the performances of a phase plate, and hence it is undesirable. This has created a demand for the development of the technology for escaping from the tradeoff such that an increase in the refractive index anisotropy results in the degradation of the wavelength dispersion characteristic.
Whereas, with a conventional technology, optical compensation films have been developed assuming that they are used mainly for 15- or less inch, small-sized, or medium-sized Liquid crystal displays. However, in recent years, it has been necessary to assume that they are used for 17- or more inch, large-sized, and high luminance Liquid crystal displays. When a conventional optical compensation film was mounted on the polarizing plate of a large-sized Liquid crystal display as a protective film, it was found that nonuniformity was created on the panel. This defect was not conspicuous very much with a small-sized or medium-sized Liquid crystal display. However, in order to cope with the trend toward a larger size and a higher luminance, there has arisen a necessity of further developing an optical compensation film capable of coping with nonuniform light leakage.
Further, with a conventional technology, the optical compensation film as described above is manufactured by coating a coating solution on a web by a coater using a slot die for lamination. In recent years, in order to develop a desirable function in manufacturing of an optical compensation film, there has been a growing demand for the coating method of a 20- or less μm area which is smaller in wet film thickness than ever. Such an optical compensation film is required to have strict coating film thickness precision and coating film properties, so that a high precision thin layer coating technique is required. Thus, a technology of forming the tip lip of a slot die into a sharp edge has been proposed (JP-T-9-511682).
On the other hand, with the method using a wire bar as the coating system of an optical compensation film, stepped nonuniformity tends to be created by coating solution vibration in the solution reservoir, and misalignment or deflection of a roll in association with coating. Further, these coating systems are post measurement systems, and hence make ensuring of the stable film thickness relatively difficult. For this reason, with these coating systems, it is difficult to increase the speed of coating to a given speed or higher. Thus, the high level of the productivity inherent in coating cannot be made full use of.